The low capital cost, short lead times and flexibility of gas turbine-based power plants make them particularly attractive to electrical utilities as a means for generating electrical power. Typical simple cycle power plants (SCPPs) generally consist of a gas turbine and an exhaust stack. The emissions from such power plants, such as nitrogen oxides (NOx), are controlled by local government regulations. Hence, if the expected NOx levels are too high to meet government regulations, the power plant design must be altered or the power plant may not be allowed to operate commercially.
One approach to reducing elevated NOx emissions is adding a selective catalytic reduction (SCR) system to the power plant that uses a reductant, i.e., ammonia, injected into the exhaust gas from the turbine which, after a catalyzed reaction, reduces the NOx emissions. Power plant applications of SCR systems for NOx reduction are typically implemented in combined cycle power plants, i.e., power plants having a gas turbine and a stream turbine and associated generators for providing an output to a power grid. In combined cycle power plants, the exhaust gas from the gas turbine is used to produce steam in a heat recovery steam generator (HRSG), commonly operating at plural pressure levels, and the steam is used as a working medium in the steam generator to thereby increase the net output of the power plant. The SCR system typically operates in a temperature range of 500° F. to 850° F., corresponding to the temperature of the flue gas provided at the exit of the HRSG.
In SCPPs, the lack of heat exchanging surfaces at the exit of the gas turbine results in temperatures that substantially exceed 850° F., i.e., higher than the optimum operating temperature for SCR systems. Operation of SCR systems at temperatures greater than 850° F. may result in unwanted chemical reactions and may shorten the expected life of the material in the SCR system. Fans have been included in the exhaust stream equipment for SCPPs to provide ambient air to mix with and cool the flue gas exiting the gas turbine. However, the fans required for supplying the cooling air add an auxiliary power requirement to the power plant. In order to optimize the power output of SCPPs, such plants are generally designed to have very low auxiliary power requirements, such that addition of auxiliary cooling fans associated with incorporation of an SCR system generally can result in a relatively large increase in the auxiliary power requirement for the plant, and an associated decrease in the net power output.